The important bit about the cluster is that each node should be monitored independently. There is no centralized node, the cluster is the set of active, connected nodes, and each can have a different view of the cluster. Further, many of these variables are relative to the node you query them from: for example, replication sent (from this node) and received (from writes on the rest of the cluster). Having data from all nodes helps tracking down the source of issues (i.e., where are the flow control messages coming from? Where did that 100MB transaction came from?).